1. Field of the Invention
The present invention relates to a servo welding gun mounted on a robot and having a welding tip driven by a servomotor, and more particularly to a method of and apparatus for detecting an abnormal load exerted on a servo gun axis on which the welding tip is provided.
2. Description of Related Art
In robotized spot welding systems, servo welding guns driven by servomotors have come to be used because of their merits such as high welding quality and short cycle time. There are two types of the servo guns as shown in FIGS. 5a and 5b, and in a single axis type servo gun shown in FIG. 5a, a fixed welding tip C1 and a movable welding tip C2 are provided at a distal end of the spot welding gun, and an axis with the movable welding tip C2 at a distal end thereof is driven by a servomotor M1 through a transmission mechanism such as a ball screw/nut. The movable welding tip C2 is moved by driving the servomotor M1 to vary a gap between the tips C1 and C2 so that an object of welding (vehicle body etc.) is clamped with pressure between the tips C1 and C2, and then spot welding is performed on the object by flowing an electric current between the tips.
In a dual axis type servo gun shown in FIG. 5b, on the other hand, both of welding tips C3 and C4 are movable and respectively driven by servomotors M2 and M3 through transmission mechanisms to vary a gap between the welding tips C3 and C4, so that an object of welding is clamped with pressure to perform the spot welding.
Each of the servomotors M1 to M3 of the above-described servo gun is controlled by a robot controller for controlling a robot with a wrist to which the servo gun is attached.
When the spot welding is performed with an objet of welding clamped with pressure between the welding tips, the welding tips are occasionally stuck to the object. In such cases, if the robot operates to move the servo gun regardless of the sticking, the servo gun moves with the object stuck thereto, possibly causing deformation of the object and a consequent secondary accident.
Conventionally, therefore, a welding machine is provided with a sticking detector for detecting sticking of the welding tips to an object of welding, and a sticking detection signal is supplied to the robot controller from the sticking detector to bring the robot to an emergency stop.
However, additional provision of the sticking detector on the welding machine leads to an increase in manufacturing cost of the system as well as in maintenance cost thereof. Further, there is known a robot with a collision detection function in which a disturbance load exerted on individual axes of the robot is detected by a disturbance estimating observer and it is judged that the robot arm or other component has collided with an obstacle when an abnormal load is detected. If this collision detection function is applied to the servo gun axis, since the servo gun axis is for clamping an object of welding between the welding tips with a predetermined pressure, the applied pressure is detected as a disturbance by the observer to output an abnormal load detection signal although the operation is normal. Therefore, this collision detection function can not be applied to the servo gun axis.
In the case where sticking of the welding tips to an object of welding is detected by the above-described collision detection function provided on the robot axes, a large estimated disturbance load is detected by the observer at an instant when the robot starts to move to the next welding point after the occurrence of such sticking. Prior to the movement of the robot, however, the welding tips of the welding gun are moved apart from each other, giving rise to a problem that the object of welding is deformed due to the opening motion of the gun. Further, if the object of welding is very soft, the disturbance load detected by the observer does not immediately increase even when the robot moves, and an abnormal load is detected only after the robot has made a large motion.